nixos/unitGenerator: fix generation for nspawn files

nixos/lib/systemd-lib.nix

@@ -214,7 +214,7 @@ in rec {
       ${concatStrings (mapAttrsToList (name: unit:
           concatMapStrings (name2: ''
             ln -sfn '${name}' $out/'${name2}'
-          '') unit.aliases) units)}
+          '') (unit.aliases or [])) units)}
 
       # Create .wants and .requires symlinks from the wantedBy and
       # requiredBy options.
@@ -222,13 +222,13 @@ in rec {
           concatMapStrings (name2: ''
             mkdir -p $out/'${name2}.wants'
             ln -sfn '../${name}' $out/'${name2}.wants'/
-          '') unit.wantedBy) units)}
+          '') (unit.wantedBy or [])) units)}
 
       ${concatStrings (mapAttrsToList (name: unit:
           concatMapStrings (name2: ''
             mkdir -p $out/'${name2}.requires'
             ln -sfn '../${name}' $out/'${name2}.requires'/
-          '') unit.requiredBy) units)}
+          '') (unit.requiredBy or [])) units)}
 
       ${optionalString (type == "system") ''
         # Stupid misc. symlinks.

nixos/tests/systemd-machinectl.nix

@@ -41,6 +41,9 @@ import ./make-test-python.nix ({ pkgs, ... }:
       systemd.targets.machines.wants = [ "systemd-nspawn@${containerName}.service" ];
 
       virtualisation.additionalPaths = [ containerSystem ];
+
+      # not needed, but we want to test the nspawn file generation
+      systemd.nspawn.${containerName} = { };
     };
 
     testScript = ''